A wide variety of coatings have been used to coat metal containing substrates such as the surfaces of packaging articles (e.g., cans) for food and beverages. For example, metal cans are sometimes coated using “coil coating” operations, i.e., a planar sheet of a suitable substrate (e.g., steel or aluminum metal) is coated with a suitable composition which is then cured. The coated substrate then is fabricated into the can end or body. Alternatively, coating compositions may be applied (e.g., by spraying, dipping, rolling, etc.) to all or a portion of can components and cured after the components already have been formed. Coatings are used both on the inside and outside of food and beverage cans. Coatings on the inside of the cans typically will directly contact the food or beverage that is packaged.
Packaging coatings should preferably be capable of high-speed application to the substrate and provide the necessary properties when cured to perform in this demanding end use. For example, the coating should be safe for food contact, have excellent adhesion to the substrate, and resist degradation over long periods of time, even when exposed to harsh environments.
Many coating compositions incorporate free radically polymerized, crosslinkable (meth)acrylic copolymers. In many instances, the copolymerization occurs in a relatively large amount of organic solvent to help provide low viscosity so that the copolymerization admixtures can be adequately stirred. Organic solvent also sometimes is used to help absorb the reaction exotherm. This increases the VOC (volatile organic content) content of the reaction admixture during manufacture. It would be desirable to reduce the VOC content of these reaction admixtures to make manufacture more environmentally friendly and to minimize the VOC of the final coating composition without practicing distillation or other technique to reduce the organic solvent content.
In some instances, coating compositions are provided in which copolymers are dispersed as suspended particles in aqueous media and/or organic solvents. Dispersions may have a tendency to prematurely dry on coating equipment. In alternative instances, coating compositions may be provided in which copolymers are dissolved in aqueous media and/or organic solvents. This avoids premature drying, and waterborne embodiments are typically low VOC.
A variety of strategies are used to synthesize copolymers that can be easily dispersed in aqueous media. One strategy incorporates a relatively large amount of (meth)acrylic acid content or the like to achieve a desired level of compatibility. Unfortunately, this can cause the Tg of the resultant copolymer to be unduly high. This can be a drawback, because this tends to increase viscosity and degrade flexibility. To lower the viscosity to a more suitable level, relatively large amounts of water and/or organic solvents may be added, but this increases the VOC and reduces NVC (nonvolatile content). Yet, if conventional strategies are used to keep the Tg at more acceptable levels, toughness and abrasion resistance tend to suffer. Strategies are desired that moderate the Tg and yet provide a high level of water compatibility without unduly sacrificing abrasion resistance and toughness.
Many current packaging coatings contain mobile or bound bisphenol A (“BPA”) or aromatic glycidyl ether compounds or PVC compounds. Such chemistry is used for many reasons, including to promote adhesion between the resultant coating and the substrate, to provide a catalyst effect, to adjust flexibility, and/or the like. Although the balance of scientific evidence available to date indicates that the small trace amounts of these compounds that might be released from existing coatings do not pose any health risks to humans, these compounds are nevertheless perceived by some people as being potentially harmful to human health. Consequently, there is a desire by some to eliminate these compounds from food contact coatings. It will be appreciated that what is needed in the art is a packaging container (e.g., a food or beverage can) that shows strong adhesion to food and beverage packaging without relying on the BPA chemistry for adhesion.